Radiofrequency (RF) tags and sensors are frequently used in national security applications, but radiofrequency sensing has numerous limitations. For instance, RF cannot penetrate metallic barriers and cannot be used in harsh environments (e.g., in water, in underground repositories, or in the presence of shelving). For safety reasons, RF cannot be used in the presence of high explosives. There are also operational hurdles, where RF tags and sensors can interfere with communications and are vulnerable to long-range interrogation by adversaries. Thus, the use of RF-based technologies can be limited for various technological, operational, or safety reasons.
Magnetoelastic tags are an alternative technology and are a familiar security measure used in stores to prevent theft. Current magnetoelastic tags are large, single frequency devices cut from strips of an amorphous magnetic material, such as Metglas®, having relatively low magnetostriction (e.g. about 12 to 30 ppm, depending on composition). These tags are typically used to announce when the tag is magnetically activated, e.g., when passing through a portal. However, the information conveyed is generally limited to an “on” or “off” response.
Magnetoelastic materials can also be used to sense a condition such as a structural flaw or a defect in a building or an aircraft. Such applications, however, are limited by a lack of commercially available materials that have high magnetostriction and are capable of providing multiplexed signals.
Accordingly, there is a need for tags and sensors that can operate in various harsh or enclosed environments, while providing multiplexed responses. Alternatives to present RF and magnetoelastic technologies having enhanced functionality and lower manufacturing costs would greatly advance tagging and sensing modalities.